This application is based on Japanese Patent Applications No. 2001-307355 filed on Oct. 3, 2001, No. 2001-308495 filed on Oct. 4, 2001, No. 2001-317688 filed on Oct. 16, 2001, No. 2001-384772 filed on Dec. 18, 2001 and No. 2002-14338 filed on Jan. 23, 2002 the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a fuel injection system and a fuel injector in an internal combustion engine (hereinafter referred to simply as engine).
2. Related Art
For example, in the case of a common rail type fuel injection system applied to a diesel engine, there usually is employed a fuel injector having a two- or three-way solenoid valve. In connection with such a fuel injector, for example the technique disclosed in JP-A-9-42106 is well known. According to this technique, fuel of a high pressure is introduced into a pressure control chamber provided on an opposite-to-nozzle holes side of a valve element, and the valve element is actuated by allowing the high-pressure fuel present in the pressure control chamber to leak to a low pressure side at every fuel injection. However, in the case of the fuel injector disclosed in the above publication, there occurs leakage of the high-pressure fuel from the pressure control chamber at every fuel injection. There also is a problem that the number of components increases and the structure becomes complicated.
Recently there has been an increasing demand for reducing the cost of the fuel injector. To meet this demand, that is, for reducing the number of components which constitute the fuel injector, a study is being made about a direct-acting type fuel injector in which a valve element is actuated directly by an electromagnetic drive unit.
On the other hand, as an alternative to gas oil and taking the volatilizability, ignitability and combustibility of fuel or emission into account, there recently has been studied the use of liquefied gas fuels such as dimethyl ether (DME) and liquefied petroleum gas (LPG) with a cetane number improving additive incorporated therein. LPG as referred to herein means a liquefied petroleum gas with a cetane number improver incorporated therein unless otherwise specified. In case of using a liquefied gas fuel, the fuel is apt to vaporize because of a low boiling point and the amount of fuel leaking from the fuel injector tends to increase. Therefore, it becomes necessary to provide a recovery system for recovering fuel leaking from the fuel injector. For example, as is disclosed in JP-A-11-22590, it is necessary to provide a purge tank for the recovery of vaporized liquefied gas fuel and a compression pump for compressing and liquefying a gaseous liquefied gas fuel recovered into the purge tank. As a result, there arises the problem that the cost of the fuel injection system concerned increases. To solve this problem, as noted above, it is proposed to use, for example, such a direct acting type fuel injector 100 as shown in FIG. 10 and thereby decrease the amount of fuel leaking from the fuel injector 100.
In the fuel injector 100 shown in FIG. 10, a valve element 101 extends vertically in the figure and an armature 102 is integrally provided at an upper end of the valve element 101 by laser welding for example. Holes 103a and 104a are formed in a casing 103 and a valve body 104, respectively, and the valve element 101 is received into the holes 103a and 104a. A stator 105 is disposed in opposition to the armature 102. When a coil 106 is energized and the armature 102 is thereby attracted to the stator 105, the valve element 101 lifts upward in FIG. 10 against the biasing force of a spring 107, whereby nozzle holes 108 are opened and high-pressure fuel fed from a common rail system is injected from the nozzle holes 108. In such a fuel injector 100 as shown in FIG. 10, the number of components is small and hence it is possible to attain the reduction of cost. Moreover, in the fuel injector 100 shown in FIG. 10, it is possible to decrease the amount of leaking fuel and therefore it becomes unnecessary to use a purge tank for the recovery of leaking fuel and a compression pump.
However, in the fuel injector 100 shown in FIG. 10, since the valve element 101 is actuated directly by an electromagnetic drive unit, it is necessary for the electromagnetic drive unit to actuate the valve member 101 against a force developed by an oil pressure acting on the valve element 101. Accordingly, for enhancing the injection pressure of fuel injected from the fuel injector 100, it is necessary to increase the size of the electromagnetic drive unit and thereby increase the driving force. However, the space ensured in an engine mounting portion is limited and therefore the size of the electromagnetic drive unit and that of the fuel injector 100 are limited. As a result, a maximum fuel injection pressure of about 30 MPa is a limit at present and a further increase of pressure is difficult.
For example, in connection with a common rail type fuel injection system for a diesel engine, there is known such a fuel injector as is disclosed in JP-A-10-18934. On the other hand, as a direct-acting type fuel injector there is proposed one illustrated in FIG. 16. In the same figure, components equal to those illustrated in FIG. 10 are identified by like reference numerals.
In an engine mounted on a vehicle, fuel injectors are replaced at every about 100,000 km running. In this case, for attaining the reduction of cost, it is proposed to remove a retaining nut 110 of a fuel injector 100 and replace only a nozzle portion 104 located at the tip of the injector. However, an armature 102 is fixed to a valve element 101 and the diameter of the armature 102 is usually larger than that of a hole 103a. This is for obtaining a satisfactory electromagnetic performance. Therefore, at the time of replacement of the nozzle portion 104, not only the removal of the retaining nut 110, but also a disassembling work for an electromagnetic solenoid portion 111 is required, resulting in that the maintainability is deteriorated. Thus, an improvement is desired.
FIG. 28 shows a fuel injector 100 in the related art. When a valve element 101 is opened, the valve member moves until abutment against a valve opening stopper 112. At this time, the valve element 101 bounces as a reaction of its abutment against the stopper 112. In many cases, for example the layout of intake/exhaust valves in an engine head portion requires the valve element 101 to be long, with the result that the valve member becomes heavy. Particularly, in the case of such a liquefied gas fuel as DME, the bounce of the valve element 101 becomes large. Such a bounce of the valve element 101 obstructs an accurate adjustment of fuel quantity.
In a fuel injector 100 shown in FIG. 33, when a valve element 101 opens, it strikes against a stopper 111 and bounces. Due to this bouncing during valve opening, an injection quantity Q becomes wavy relative to a pulse width T, thus making injection control difficult.
Further, when a coil 106 is de-energized, with loss in attraction of an armature 102 by a stator 105, and the valve element 101 closes with the biasing force of a spring 107, the valve element 101 strikes against a sheet portion of a nozzle body 104 and causes bouncing. Due to this bouncing in valve closing, there occurs re-injection (secondary injection) after the end of injection, thus resulting in deterioration of the injection characteristic.
On the other hand, in many cases, the valve element 101 is required to be long for example due to the layout of intake/exhaust valves in an engine head, resulting in that the valve element 101 becomes heavy and there occurs markedly such bouncing as referred to above.
Particularly in the case of such liquefied gas fuels as LPG and DME, since their viscosities are low, not only the bouncing of the valve element 101 becomes large, but also the time taken until damping of the bounding becomes long and the aforesaid inconvenience occurs markedly.
A leak fuel recovery system is disclosed, for example, in JP-A-11-22590. An outline thereof will now be given with reference to FIG. 35. In the same figure, fuel stored in a fuel tank 550 is discharged from a low pressure pump 551 and is compressed to a high pressure by means of a high pressure pump 552, then is fed to a common rail 553. Connected to the common rail 553 are fuel injectors 554 in a number corresponding to the number of engine cylinders.
Fuel leaking from the high pressure pump 552 and fuel injectors 554 is once recovered into a fuel recovery tank (purge tank) 555, then is liquefied by a fuel compressor 556 and is returned to the fuel tank 550.
In the construction of FIG. 35 it is necessary to provide a leak fuel recovery system comprising the fuel recovery tank 555 and the fuel compressor 556, thus giving rise to the problem that the construction becomes complicated and the cost increases.
It is an object of the present invention to provide an improved fuel injector.
It is another object of the present invention to provide a fuel injector having a compact construction and capable of handling high pressure fuel.
It is a further object of the present invention to provide a fuel injector improved in maintainability.
It is a still further object of the present invention to provide a fuel injector wherein the bouncing of a valve member is suppressed.
It is a still further object of the present invention to provide a liquefied gas fuel supply system having a high utility.
In one aspect of the present invention there is provided a fuel injector which is provided with an oil pressure reducing means. The oil pressure reducing means reduces an oil pressure acting in a nozzle hole closing direction which oil pressure is included in an oil pressure acting on a valve element. Since the oil pressure acting on a valve element in the nozzle hole closing direction is reduced, the force required for an electromagnetic drive unit to actuate the valve element decreases. Consequently, even when the valve element is actuated directly by the electromagnetic drive unit, the pressure of fuel fed to the fuel injection system concerned can be increased while retaining the constitution of the electromagnetic drive unit for example. Thus, even when the valve element is actuated directly by the electromagnetic drive unit, the pressure of injected fuel can be further increased without an increase in size of the constitution.
The above fuel injector according to the present invention is what is called an actuator direct acting type fuel injector wherein an armature is attracted to a stator upon energization of a coil and consequently a valve element integral with the armature moves to open the nozzle hole. In this construction, the valve element is provided in a divided manner into a rod portion and a valve portion, which are connected together through a connecting member. According to this construction, when the armature is attracted to the stator upon energization of the coil, the valve portion moves together with the rod portion to open or close the nozzle hole. With the rod portion, the valve portion and the connecting member connected to one another, the rod portion is accommodated in a first casing and the valve portion is accommodated in a second casing.
According to the above construction, if the first and second casings are disassembled and the connecting member is disconnected, it becomes possible to remove only the valve portion exclusive of the rod portion. Therefore, when the valve portion is to be replaced after a long-term use of the fuel injector, the replacing work efficiency is improved. As a result, it is possible to realize a construction superior in maintainability of an actuator direct acting type fuel injector.
In the above construction, when the coil is energized, the armature is attracted to the stator against the biasing force of a spring and the valve element moves to its closing position. In this case, since an oil pressure damper chamber is provided between an end face of the armature and that of the stator, the bouncing of the armature and valve element is suppressed when the valve opens by virtue of a damper effect. Therefore, it is possible to keep the fuel injection quantity under control.
According to the present invention, when an electric actuator (e.g., an electromagnetic solenoid or a piezo-electric actuator) causes an armature (driver) to displace in the valve opening direction, fuel having an accumulated pressure is injected from a nozzle. As a result of this injection, the pressure decreases on the nozzle side rather than in a throttle portion and the pressure in a second chamber becomes lower than that in a first chamber. Since the second chamber lower in pressure lies on the side (in the valve opening direction) opposite to the nozzle, a pressure receiving portion is urged to the opposite-to-nozzle side (in the valve opening direction) by virtue of a differential pressure. With this urging force based on the differential pressure, the bouncing of the valve element when opened is suppressed. When the electric actuator causes the armature to displace in the valve closing direction, the injection of fuel is stopped. Once the fuel injection is stopped, the flow of injected fuel is cut off suddenly, so that the pressure on the nozzle side rather than in the throttle portion increases to a higher level than the pressure of accumulated pressure fuel and the pressure in the second chamber becomes higher than that in the first chamber. At this time, the first chamber which is low in pressure lies on the nozzle side (in the valve closing direction), so that the pressure receiving portion is urged to the nozzle side (in the valve closing direction) by virtue of a differential pressure. With this urging force induced by the differential pressure, the bouncing of the valve element when closing is suppressed. Since the bouncing in valve opening and closing is thus suppressed, the injection characteristic is improved. Even in the case where the valve element is long and heavy, it is possible to improve the injection characteristic because the occurrence of bounce is suppressed by the differential pressure.
Further, even where the fuel viscosity is low as in such a liquefied gas fuel as LPG or DME, since the occurrence of bounce is suppressed by the differential pressure, it is possible to improve the injection characteristic.
According to a further feature of the present invention, fuel having an accumulated pressure is injected from the nozzle upon displacement of the armature in the valve opening direction by the electric actuator. With this fuel injection, the fuel flows from the first chamber to the second chamber formed on the side (in the valve opening direction) opposite to the nozzle through a passage formed along the side face of the armature. As a result of this fuel flow in the valve opening direction, the armature undergoes a force advancing toward the side (in the valve opening direction) opposite to the nozzle, whereby the bouncing of the valve body in valve opening is suppressed. When the electric actuator causes the armature to displace in the valve closing direction, the injection of fuel is stopped. Once the fuel injection is stopped, the flow of the injected fuel is cut off suddenly, so that the pressure on the nozzle side rather than in the throttle portion rises to a higher level than that of the accumulated pressure fuel which is fed and the pressure in the second chamber becomes higher than that in the first chamber. As a result, the fuel flows from the second chamber which is high in pressure to the first chamber located on the nozzle side (in the valve closing direction) through the passage formed along the side face of the armature. With this fuel flow in the valve closing direction, the armature undergoes a force advancing toward the nozzle side (in the valve closing direction), so that the bouncing of the valve element when closing is suppressed.
In another aspect of the present invention there is provided a fuel supply system for the supply of a liquefied gas fuel, in which a liquefied gas fuel stored in a fuel tank is fed through fuel piping to a fuel injection system.
In this system there is provided an air conditioner which is provided with at least an expansion valve, an evaporator, and a condenser, and a liquefied gas fuel stored in the fuel tank is fed as refrigerant to the air conditioner. Further, the liquefied gas fuel leaking from the fuel injection system is introduced into the air conditioner.
The liquefied gas fuel introduced into the air conditioner is mixed as refrigerant into the liquefied gas fuel which is circulating through the air conditioner, then flows downstream.
According to the above construction, the liquefied gas fuel leaking from, for example, a high pressure pump and a fuel injector both constituting the fuel injection system is subjected to a liquefying process in the air conditioner (condenser) and is returned to the fuel tank through the air conditioner. Thus, there is not required any additional construction as the fuel recovery system. Additionally, the condenser in the air conditioner plays the role of recovering the leak fuel in addition to its inherent role of liquefying the refrigerant (liquefied gas fuel) and thus the condenser can be used in common. As a result, it is possible to simplify the construction of the fuel supply system and reduce the cost thereof.